1. Field of the Invention
This invention relates to the upgrading and cracking of heavy hydrocarbons to lighter, fuel range liquids. More specifically this invention relates to the use of a water containing medium as a means for cracking heavy hydrocarbons in the presence of light olefins in order to improve the yields of liquid products and decrease the yield of gaseous products.
2. Description of the Prior Art
Cracking is a refining process involving the decomposition and molecular recombination of organic compounds, especially hydrocarbons obtained by the distillation of petroleum, by means of heat, to form molecules suitable for motor fuels, monomers and petrochemicals. In the cracking process, a series of molecular reactions take place accompanied by the transfer of hydrogen atoms between molecules which brings about fundamental changes in their structure.
There are two major types of cracking operations. Thermal cracking exposes petroleum distillate to temperatures of about 540.degree. C. to 650.degree. C. for varying periods of time. Catalytic cracking employs a metallic catalyst over which hydrocarbon vapors are passed to about 400.degree. C.
In both thermal and catalytic cracking processes steam has been used as a diluent for the organic matter. Typically, the cracking processes employing steam operate at between 50 and 150 psig.
In part, the instant invention pertains to dense, water containing fluids at elevated temperatures and pressures, used as a cracking medium. More specifically, the instant invention pertains to the use of supercritical fluids as a cracking medium. A fluid which is at both a temperature and pressure exceeding its critical temperature and pressure is a supercritical fluid. A supercritical fluid exists as a form of matter in which its liquid and gaseous states are indistinguishable from one another. The critical temperature of a fluid is the temperature above which that fluid cannot be liquified by an increase in pressure. The critical pressure of a fluid is simply the pressure of the fluid at its critical temperature.
Of particular interest in the instant invention are dense water containing fluids at supercritical conditions used as a cracking medium. Water is a supercritical fluid when its temperature and pressure exceed 374.degree. C. and 3204 psi (218.3 atm). The conditions and properties of this water containing fluid places it outside the scope of steam.
Lastly, the instant invention pertains to the use of light olefins in the supercritical fluid cracking process. Light olefins have been added to thermal and catalytic cracking processes in order to serve as a diluent or to drive the reaction equilibrium away from the formation of gaseous olefins. The instant invention seeks to achieve a significant increase in the amount of liquid range products from the supercritical fluid cracking of heavy hydrocarbons by the addition of light olefins.